Halal cart food?
There's a Metro Halal cart in Rosslyn, as well, in front of 1100 Wilson Blvd (under the big tv screen) - but that's not closer to Falls Church/Vienna. What I can say is that he's normally still out there when I leave from work at 5pm, so you could maybe get dinner after work...
if you ever make it to baltimore...
Halal food cart (corner of West Baltimore and North Liberty street) - For $6 bucks, you can get a huge portion of either chicken or lamb gyro meat on rice with the famous "white" or "red" sauces (drink included). This is all I've tried here and it's similar to what you'll find in New York City carts that made this famous. Though it isn't as tasty as those, mainly because of the cumin amount in the rice, it's something I look forward to and eat about once a week. They also have gyros and sandwiches which are cheaper (around $4).
Does it have to be from a cart?
Halalco Supermarket in Falls Church has a cheap buffet with average (very, very average) chickpeas, pakistani chicken curries, rice, and so forth. Its spicy and oily and halal....could satisfy a cart craving, but thats about it. The same can be said of Lahori Kabob, also in Falls Church... its dingy as heck, oily pakistani border buffet food, but its cheap, its halal, and the fresh naan with the meal is solid.
No carts that I know of though.